1. Field of the Invention
The present invention relates to an image pickup apparatus and, more particularly, is suitably applied to an image pickup apparatus for determining an appropriate exposure amount from output information from an image pickup means.
2. Related Background Art
FIG. 1 is a block diagram showing a schematic arrangement of a conventional image pickup apparatus having a solid-state image pickup element as an image pickup sensor.
Referring to FIG. 1, image pickup light from an object to be photographed passes through an image pickup optical system 1 and is guided to an image pickup element 3 serving as an image pickup sensor through an iris 2. The light is converted into an electrical signal by this image pickup element 3.
Image information represented by the converted electrical signal is input to an A.D (analog-to-digital) converter 6 through a sample-and-hold (SH) circuit 4 and a clamp (CL) circuit 5. The image information is converted into a digital signal by the A.D converter 6. The digital signal is stored in a memory 7.
A digital signal processor (DSP) 8 reads out image information stored in the memory 7 and performs predetermined signal processing. The processed data is sent to a D.A (digital-to-analog) converter 9.
The image information converted to an analog signal by the D.A converter 9 is sent to a recording device (not shown) through an output terminal T.sub.OUT and is recorded as a video signal. At this time, the respective components after the image pickup element 3 are operated in synchronism with timing signals output from a synchronization signal generator (SSG) 30. The operations of the synchronization signal generator 30, the iris 2, and a photometry sensor 31 are controlled by a system controller 32.
The above image pickup apparatus comprises the exclusive photometry sensor 31. The photometry sensor 31 measures the luminance of the object to be photographed, and the measurement result is transmitted to the system controller 32. The system controller 32 calculates an appropriate degree of opening of the iris 2 and a storage time of a signal charge of the image pickup element 3 on the basis of the above measurement result. The system controller 32 then controls the iris 2 in accordance with the calculated values and at the same time causes the synchronization signal generator 30 to control the image pickup element 3. Therefore, a good image of the object can be obtained in an appropriate exposure amount.
When a sufficiently large exposure amount cannot be obtained due to a low object luminance, an auxiliary light device 50 is operated. The auxiliary light device 50 sends a status signal to the system controller 32 to determine whether light can be generated by the auxiliary light device 50 when the device 50 is started to operate. When the system controller 32 determines that auxiliary light can be generated in an exposure state of a low object luminance, an auxiliary light emission signal is sent from the system controller 32 to the auxiliary light device 50.
A modulated light control device 51 stops light emission of the auxiliary light device 50. The modulated light control device 51 integrates light from the object from the start of light emission of the auxiliary light device 50 and at the same time sends a light emission stop signal for the auxiliary light when the integration value reaches a predetermined value. Upon reception of the light emission stop signal, the auxiliary light device 50 stops light emission.
The above image pickup apparatus has a separate photometry sensor. In recent years, a technique for repeatedly reading out and integrating signals from the image pickup sensor (image pickup element) prior to exposure and causing a system controller 32 to determine an integration result to calculate an appropriate iris value and a charge storage time has been proposed.
Examples of the above technique are disclosed in Japanese Laid-Open Patent Application Nos. 1-320872 and 1-320876. In addition, methods using special sensors are also proposed in Japanese Laid-Open Patent Application Nos. 57-95771 and 58-15375.
In a conventional image pickup apparatus of the type described above, a photometry sensor is required in a normal operation. For this reason, means for adjusting a sensitivity difference between photometry and image pickup sensors, a difference between a light reception angle and a screen angle, and a difference in spectral sensitivity are required. The resultant arrangement is complicated and expensive. At the same time, a space is required for mounting a photometry sensor resulting in a bulky apparatus.
When an image pickup sensor also serves as a photometry sensor, photometry is performed by the above sensor to determine an iris value and a sensor charge storage time, and then exposure is performed. The image pickup sensor cannot therefore cope with a difference in object luminance between the photometric operation and the exposure operation. For this reason, when an object having a high luminance is moved in or out of the image pickup area upon the start of exposure, or an object is a light-emitting element which changes such as are which flickers, blooming occurs due to the signal charge overflow in the sensor. A decisive problem may occur in addition to slight overexposure or underexposure.
In Japanese Laid-Open Patent Application No. 57-95771, a nondestructive read-out image pickup element which can cope with a change in object luminance after the start of exposure is used. However, even if the nondestructive read-out element is used, destructive read-out must be performed to obtain an image having a practically sufficiently high quality. Therefore, this element cannot be used in practical applications without any modifications.
In order to achieve the above arrangement, an image pickup element must have an exclusive photometry read-out circuit.
An exclusive photometry read-out circuit is required in Japanese Laid-Open Patent Application No. 58-15375 as in Japanese Laid-Open Patent Application No. 57-95771. In addition, in this case, some pixels are read out for photometry. Of all image data read out upon completion of the photometry, data of pixels having already been read out for photometry are displayed in black, resulting in inconvenience.